1. Field of the invention
The present invention relates to electroacoustical transducers in general, and to vibratile diaphragms in such transducers, in particular.
2. Description of the Prior Art
Capacitance type electroacoustical transducers are well known in the prior art. In such transducers, a diaphragm having an insulative layer and an electrically conductive surface has its insulative layer in contact with a grooved, irregular, electrically conductive surface of a substantially inflexible disc or backplate. The periphery of the diagram is maintained in a fixed position with respect to the transducer housing and a spring force urges said backplate into tensioning engagement with said diaphragm. The insulative layer, the electrically conductive surface of said diaphragm constituting a first electrode, and the conductive surface of said backplate constituting a second electrode, form a capacitor such that when a dc bias voltage is applied across said electrodes, irregularities in said backplate surface set up localized concentrated electric fields in said insulative layer. When an ac signal is superimposed on said dc bias, the insulative layer is stressed such that oscillatory formations develop causing an acoustical wavefront to be propagated from the diaphragm. A received acoustical wavefront impinging on the insulative layer produces a variable voltage across said capacitor electrodes.
An extremely important design consideration for the above-described transducer is the amount of tension in the transducer diaphragm. In addition to such factors as resonant frequency and output magnitude, diaphragm tension also affects transducer sensitivity in at least two ways. Within limits, less diaphragm tension provides greater reception sensitivity. Also, incorrect diaphragm tension may introduce stress patterns into the diaphragm causing said diaphragm to wrinkle which will affect the ability of the diaphragm to uniformly contact a backplate surface. Such nonuniform diaphragm contact will directly affect transducer efficiency and therefore indirectly affect transducer sensitivity. When a wrinkled diaphragm nonuniformly contacts said plackplate surface, those wrinkled diaphragm areas that are spaced a significant distance from said surface will produce less capacitance change per unit of diaphragm movement from a received acoustical wavefront, or cause a lower magnitude wavefront to be propagated during transmission, than those diaphragm areas that are not so spaced from said backplate surface. This is of more concern in the reception of an acoustical signal where signal levels tend to be low than in the transmission of a signal where signal levels tend to be relatively high. This is also of more concern in a small electroacoustical transducer whose sensitivity is necessarily low from its smaller size than in a large electroacoustic transducer with its larger transducer components.
Prior art electroacoustical transducers have their diaphragms peripherally clamped and have either zero or a predetermined amount of tensioning force on said diaphragms prior to diaphragm/backplate engagement. In such transducers, diaphragm tensioning is either introduced or increased by properly mating the backplate to the diaphragm. This type of diaphragm tensioning introduces at least some sensitivity reducing diaphragm wrinkles, especially when the backplate has a raised center portion that is sometimes referred to as a crown. If diaphragm wrinkles can be reduced or eliminated from the diaphragm/electrically conductive backplate surface interface, improved transducer efficiency and sensitivity will result.